1. Field of the Invention
The present invention relates to a digital data transmission technique, and more particularly to a multi-carrier transmission system that utilizes an orthogonal frequency division multiplexing (OFDM) transmission technique using a multi-carrier.
2. Description of the Related Art
In OFDM as a type of multi-carrier transmission system, a transmitter multiplexes frequency-base signals into time-base signals using an inverse discrete Fourier transformer (IDFT), while a receiver extracts frequency-base signals from received time-base signals using a discrete Fourier transformer (DFT). No further particulars of OFDM will be explained since OFDM is a well-known technique.
When an OFDM receiver receives a transmission signal, a DFT performs block processing. Accordingly, it is necessary to accurately set the positions of blocks, i.e., to perform symbol synchronization. In general, to realize symbol synchronization, a transmitter adds a redundant symbol to a to-be-transmitted signal.
There is a method in which a guard symbol is inserted and synchronization is performed based on the symbol. Specifically, if, for example, there is eight IDFT outputs x0, x1, . . . , x7, the last four outputs x4, x5, x6 and x7 are copied, and the four copies are positioned before the original four outputs and used as a guard symbol. The thus-obtained outputs, twelve outputs in total, are transmitted as one symbol. When a receiver executes correlation computation on each pair of adjacent ones of the twelve outputs, it finds that the four points of the guard symbol and the four outputs positioned after the guard symbol show high correlation values, since the four guard symbol points are copies of the four outputs. From this, a symbol synchronization position can be specified. (See, for example, Jpn. Pat. Appln. KOKAI Publication No. 7-99486)
Further, Jpn. Pat. Appln. KOKAI Publication No. 2003-69546, for example, discloses a technique for transmitting, as a preamble, a known signal for synchronization, and making a receiver detect the known preamble as a symbol synchronization position.
A block signal as an IDFT output signal from a transmitter is used as a transmission symbol. The transmitter continuously transmits the transmission symbol. A receiver accurately detects the leading portion of the transmission symbol by detecting, for example, a preamble contained therein. Thus, the receiver performs symbol synchronization, and then inputs each symbol to a DFT to perform signal reproduction.
When the transmission channel has multipath characteristics, the receiver receives delayed waves as well as direct waves. Accordingly, when synchronization is established at the leading portions of direct waves, inter-symbol interference in which present and preceding symbols are mixed occurs. In the prior art, to eliminate such inter-symbol interference, a transmitter inserts, for example, a guard symbol in each symbol to be transmitted. Assuming, for example, that the output signals of an IDFT with eight input/output points are x0, x1, . . . , x7, the last four outputs X4, x5, x6 and x7 are copied, and the four copies are positioned before the original four outputs and used as a guard symbol. The thus-obtained outputs, twelve outputs in total, are transmitted as one symbol. In this case, if a multipath delay is within a time corresponding to four outputs, the time of inter-symbol interference is limited to the period of the guard symbol. Therefore, if x0, x1, . . . , x7 are input to the DFT with eight input/output points, signal reproduction with suppressed inter-symbol interference can be performed. Further, if the multipath delay is longer than the above, the number of guard symbols may be increased (See, for example, Jpn. Pat. Appln. KOKAI Publication No. 2002-374223).
A block signal as an IDFT output signal from a transmitter is used as a transmission symbol. The transmitter continuously transmits the transmission symbol. A receiver accurately detects the leading portion of the transmission symbol by detecting, for example, a preamble contained therein. Thus, the receiver performs symbol synchronization, and then inputs each symbol to a DFT to perform signal reproduction.
In OFDM transmission, the range of amplitude variation is large, therefore non-linear distortion may easily occur. Accordingly, a receiver for performing OFDM transmission needs to have an analog receiving circuit of high linear performance that can receive, without distortion, signals having significantly different amplitudes, or needs to perform control for suppressing the maximum amplitude of a transmitter output (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2003-46480).
When an OFDM receiver receives a transmission signal, accurate setting of the block position, i.e., symbol synchronization, is indispensable since a DFT employed therein performs block processing. In general, to realize symbol synchronization, a transmitter adds a redundant symbol to a signal to be transmitted.
For example, there is a method in which a guard symbol is inserted, and synchronization is performed using this symbol. Specifically, assuming, for example, that the output signals of an IDFT with eight input/output points are x0, x1, . . . , x7, the last four outputs x4, X5, x6 and x7 are copied, and the four copies are positioned before the original four outputs, and used as a guard symbol. The thus-obtained outputs, twelve outputs in total, are transmitted as one symbol. When a receiver executes correlation computation on each pair of adjacent ones of the twelve outputs, it finds that the four points of the guard symbol and the four outputs positioned after the guard symbol show high correlation values, since the four guard symbol points are copies of the four outputs. From this, a symbol synchronization position can be specified. (See, for example, Jpn. Pat. Appln. KOKAI Publication No. 7-99486)
There is another method in which a known signal for synchronization is transmitted as a preamble, and a receiver detects the preamble to detect the symbol synchronization position (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2003-69546).
In the above-described multi-carrier transmission system, however, symbols for synchronization must be inserted to enable a receiver to perform symbol synchronization, which reduces the transmission efficiency.
Further, the above-described method for increasing the number of guard symbols is disadvantageous in that the transmission efficiency is inevitably reduced.
Concerning the analog receiving circuit of high linear performance, this circuit is expensive, therefore the use of the circuit inevitably increases the cost of the communication system. If the maximum amplitude of the transmitter output is suppressed, the feature of the OFDM transmission system cannot sufficiently be utilized.